1. Field of the Invention
The present invention relates to a method for operating a hybrid vehicle.
2. Description of Related Art
An electromagnetic power-branching hybrid drive having at least two units, one internal combustion engine and one electric machine, are known and permit operating the internal combustion engine in a high efficiency range.
The driving power of the internal combustion engine is divided up into a power component, using one or more mechanical override transmissions (planetary transmissions), which becomes a drive purely mechanically, and therefore at high efficiency. The remaining power is converted into electric power via a regeneratively functioning electrical machine, and is mostly fed back into the transmission by a second motor-type electric machine.
Because of the override properties of the planetary transmission, while ignoring elasticities, there is typically one rotational speed degree of freedom available. Thus, at a specified vehicle speed and a specified gear, if several gears may be selected, the rotational speed may be freely selected within physical limits. Transmission constructions are known in which the rotational speed of the second electric machine depends directly on the vehicle speed. In other concepts, this is the case at least in one of several gears. In response to changes in the rotational speed degree of freedom, that is, in the rotational speed of the internal combustion engine, there comes about a change in rotational speed at the first electric machine, but the rotational speed of the second electric machine remains constant with the vehicle speed.
In hybrid operation, the available rotational speed degree of freedom is used to operate the power train in the range of high overall efficiencies. A control system takes into account essentially the vehicle speed and the actual rotational speed of the transmission output shaft, the tractive force requested by the driver and the power required to supply the vehicle electrical system, and based on these, it determines the rotational speed degree of freedom available, as well as the torques of the units.
In the known attempts to regulate such a power train having one rotational speed degree of freedom, only one unit, usually one of the two electric machines, is operated with its rotational speed regulated, that is, a comparison of the setpoint rotational speed to the actual rotational speed takes place for only one unit.
The two other units, the second electric machine and the internal combustion engine, are torque-controlled, partially as a function of the rotational speed deviation at the rotational speed-regulated unit.
In published U.S. patent application documents US 2002/0113440 A1 and US 2003/006094 A1, in each case one of the two electric machines is rotational speed-regulated. A setpoint rotational speed of the internal combustion engine is calculated which, together with the vehicle speed, is used to ascertain the setpoint rotational speed for one of the two electric machines, which is then rotational speed-regulated by comparison of its setpoint rotational speed to its actual rotational speed.
Published U.S. patent application document US 2002/0024306 A1 proposes only regulating the rotational speed of the internal combustion engine in the power-branching operation. The setpoint rotational speed of the internal combustion engine is compared to its actual rotational speed, and based on this, an intervention takes place in torques.